1. Field of the Invention
The present invention relates to a semiconductor device adopting micro-strip line conductors as input line conductors of a high-frequency semiconductor element, and specifically to a high-gain semiconductor device that can reduce the reflection of input signals.
2. Background Art
A high-frequency semiconductor element for upper surface mounting that operates in a high-frequency band of MHz or GHz bands has been used. Normally, this high-frequency semiconductor element is mounted on a dielectric substrate composed of an epoxy resin or the like. For the input/output line conductor of the high-frequency semiconductor element, a micro-strip line conductor is often adopted from the point of view of ease of electrical design and area reduction (see, for example, Japanese Patent Laid-Open No. 2004-214584).
A conventional semiconductor device wherein a semiconductor package is mounted on a mounting substrate using a micro-strip line conductor in input/output line conductors will be described. FIG. 32 is a sectional view showing a conventional semiconductor device. FIG. 33 is a top view showing a conventional mounting substrate, and FIG. 34 is a top view showing a conventional semiconductor package.
The mounting substrate 11 has input/output line conductors 14 formed on the upper surface of a substrate body 13; an upper surface grounding conductor 15 formed on the upper surface of the substrate body 13 apart from the input/output line conductors 14; and a lower surface grounding conductor 17 formed on the lower surface of the substrate body 13 and electrically connected to the upper surface grounding conductor 15 through through-holes 16. The input/output line conductors 14 and the lower surface grounding conductor 17 form micro-strip line conductors.
The semiconductor package 12 has input/output terminals 21 composed of metal frames electrically connected to the end portion of the input/output line conductors 14, a grounding terminal 22 electrically connected to the upper surface grounding conductor 15, a high-frequency semiconductor element 24 die-bonded on the grounding terminal 22 and electrically connected to the input/output terminals 21 through bonding wires 23, and a molding resin 25 for sealing these.
The semiconductor package 12 is positioned in a self-aligning manner by the surface tension of solder when mounted on the mounting substrate 11. In order to make the surface tension work effectively, the conventional upper surface grounding conductor 15 is rectangular, and the upper surface grounding conductor 15 on the mounting substrate 11 is designed to be slightly larger (about 0.1 to 0.2 mm) than the grounding terminal 22 of the upper surface-mounted semiconductor package 12.
Here, a case wherein input signals are propagated into a conventional semiconductor device from the left of FIG. 32 is considered. Since the cross section of the micro-strip line conductor formed from the input/output line conductors 14 and the lower surface grounding conductor 17 is uniform from the cross section A-A′ to cross section B-B′, the input signals are propagated without reflection. The lower surface grounding conductor 17 and the input/output terminals 21 or the lower surface grounding conductor 17 and bonding wires 23 form the micro-strip line conductor from the cross section B-B′ to cross section D-D′. Since the line impedance in this portion is different from the line impedance from the cross section A-A′ to cross section B-B′, slight reflection occurs; however, the propagation mode of input signals is same as the propagation mode of the micro-strip line conductor, and serious problems do not normally arise.
However, the grounding upper surface shifts vertically from the lower surface grounding conductor 17 to the grounding terminal 22. Therefore, the advancing direction of the input signals changes from the lateral direction to the upward direction of FIG. 22, the propagation mode of input signals is no longer the same as the propagation mode of the micro-strip line conductor. Consequently, there was a problem wherein input signals were significantly reflected at the cross section D-D′, reducing the gain of the semiconductor device.